Georges Guiochon

UTK chemistry

Separation science: Determining chemical processes in living organisms

Living cells, going about their daily business, metabolize food into fats, which they store to satisfy further energy needs, and provide other vital cell-building chemicals.15 The process involves nearly 1000 metabolites (low molecular weight compounds).

Scientists have been investigating the cells that make and store fats, to see if they might be harvested for biodiesel fuels. Algae are particularly attractive because they do not make cellulose but only fats and need only water, sunlight, and a few minerals to survive. But researchers need to understand the details of the metabolism of these fats if they are to select the algae with the highest productivity and the most favorable conditions for their culture.

Georges Guiochon says, "The complexity of these mixtures of metabolites defies the most powerful combinations of analytical methods currently available."

A single simple step in the metabolism of fatty acids, triglycerides, glucose, or cholesterol involves several hundred different metabolites. Unraveling the successive steps involved requires identification and quantitative analysis of each.

Biochemists and molecular biologists turn to liquid chromatography (HPLC) to fractionate the mixtures of compounds isolated from cells. HPLC percolates a steady stream of solvent through a long tube packed with fine adsorbent particles. Samples are injected at the column inlet, and their composition monitored at the exit. Given the proper solvent and adsorbent, each compound will migrate along the column at its own velocity, depending upon the molecular interaction with the adsorbent surface. The eluted fractions are analyzed by mass spectrometry (MS), which identifies the molecular structure, and/or nuclear magnetic resonance (NMR), which identifies spatial relationships within individual compounds.

But each of these methods has limitations, says Guiochon. MS requires relatively simple mixtures of six to 12 compounds; NMR is only useful if the sample is pure. HPLC will separate complex mixtures into fractions simple enough to be identified with these spectrometric methods but it is limited to a few hundred chemicals per column.

To address the problem, Guiochon's group couples together two successive separations.

This can be done in a number of ways. The most powerful collects a few hundred fractions at the exit of one column and separates these fractions on a second column, according to different characteristics. Success requires the development of fast methods of chromatography, which recently led to a rapid evolution of chromatographic column technology. The new columns are packed with finer and finer porous particles, which makes them faster and more powerful; but this also increases the inlet pressures, and temperatures rise from the friction of the liquid stream percolating through the column. The escaping heat cools the column wall faster than the center. As a result the center stream is faster than flow at the wall because heat decreases viscosity. So, the bands of compound leaving the column are not uniform.

Guiochon's investigations found a solution to this problem. They replaced the conventional packing with particles that have a solid core surrounded by porous shell. The chromatographic properties of the shell are the same as the fully porous ones, but they conduct heat out from the center far more quickly, reducing the loss in efficiency.

A leading expert in multidimensional chromatography, Guiochon received the 2010 Lifetime Achievement Award for contribution to separation science from the LCGC Magazine at the recent Pittsburg conference and the 2009 Professor Andrzej Waksmundzki Medal from the Polish Chemical Society.

You can read more about Guiochon's research on his Department of Chemistry web page, http://www.chem.utk.edu/faculty/guiochon.html.